In an auxiliary machine driving unit of a vehicle or the like, which drives an auxiliary machine such as an alternator by power of an engine, a belt is stretched across a pulley linked to a driving shaft of the auxiliary machine such as an alternator, and a pulley linked to a crankshaft of the engine, and torque of the engine is transmitted to the auxiliary machine via the belt. As the pulley linked to the driving shaft of the auxiliary machine such as an alternator, for example, a pulley structure of Patent Document 1 which can absorb rotation variation of the crankshaft.
The pulley structure disclosed in Patent Document 1 includes a first rotating body around which a belt is wound, a second rotating body that is provided on an inner side of the first rotating body and can relatively rotate with respect to the first rotating body, and a coil spring disposed in a space (hereinafter, referred to as a spring accommodation space) formed between the two rotating bodies. The spring accommodation space is defined by, in addition to the two rotating bodies, an end cap that blocks an opening portion on a front side of the first rotating body and a rolling bearing installed to be interposed between a rear end portion of the first rotating body and the second rotating body.
Generally, as the rolling bearing of the pulley structure provided in the driving shaft of the auxiliary machine such as an alternator, use is made of a contact seal type-sealed ball bearing in which grease (hereinafter, referred to as bearing grease) is sealed. The contact seal type-sealed ball bearing includes an outer wheel, an inner wheel disposed at an inner circumference of the outer wheel, a plurality of balls disposed to be freely rollable between the inner wheel and the outer wheel, and an annular contact seal member disposed on both sides of the plurality of balls. The contact seal member is formed of a rubber-like elastic body and sheet metal. An outer circumferential edge of the contact seal member is fixed to the outer wheel, and a lip portion formed at an inner circumferential edge of the contact seal member comes into contact with a sealing surface of the inner wheel to be pushed and widen. Therefore, the contact seal type-sealed ball bearing has excellent dust resistance and waterproof properties, and the bearing grease can remain for a long period of time.
In the contact seal type-sealed ball bearing, a slight void exists between the lip portion of the contact seal member and the sealing surface of the inner wheel. Friction and wear are caused or deformation is caused by a pressure difference between the inside and the outside of the bearing, pressure due to residual grease that does not contribute to lubricating, or sliding or vibration between the lip portion and the inner wheel. The above-described void is thus generated. As the slight void exists, the contact seal type-sealed ball bearing has a structure into which foreign substances or moisture are unlikely to infiltrate from the outside but allows ventilation.